Salmonella enterica serovar Typhimurium (Salmonella Typhimurium) is a common cause of enterocolitis in humans and cattle but causes a systemic, typhoid-like, disease in susceptible mice. This facultative intracellular pathogen invades non-phagocytic cells, particularly intestinal epithelial cells. Interactions with epithelial cells are largely mediated by two type III secretion systems (T3SS1 and T3SS2), which translocate distinct cohorts of bacterial effector proteins into the host cell. Following internalization, which is mediated effectors translocated by T3SS1, the bacteria are localized within a modified phagosome, known as the Salmonella-containing vacuole (SCV). Maturation of the SCV is mediated by effectors translocated by T3SS2. Intracellular replication occurs within the SCV and within the cytosol, bacterial growth rates and transcriptional status within these two intracellular niches are quite different. In particular, the T3SS1 is functionally active in the cytosolic bacteria whereas the T3SS2 is functionally active in the vacuolar bacteria. Thus, each of the T3SS systems are functional in the intracellular environment albeit in different niches. We are dissecting the roles of these two secretion systems, particularly T3SS1, both in vitro and in vivo.